The measurement of TOC has become a standard method for assessing the level of organic contaminants in purified water. This assessment is necessitated by the demand for purified water in industry and in such closed environments as spacecraft, which have water recycling systems. These systems include components such as the TOC analyzer of the present invention to monitor the quality of the water.
The TOC concentration of a sample of water is generally determined by quantitative analysis of the carbon dioxide generated when organic compounds are oxidized. Several approaches have been disclosed in the prior art. Examples of these approaches are contained in the U.S. Pat. Nos. of Carlson (4,209,299), Ezjak (4,277,438), Blades et al. (4,666,860) and Godec et al. (5,132,094).
Carlson teaches a method of measuring the concentration of a volatile electrolyte, such as carbon dioxide, in a liquid. Carlson's method is to pass a sample stream and a stream of known conductivity on opposite sides of a gas-permeable, liquid-impermeable membrane. The volatile electrolyte in the sample passes through the membrane and into the second liquid. The amount of the volatile electrolyte in the second liquid is then determined by the change in the liquid's conductivity.
Ezjak discloses a similar use of a gas-permeable membrane, but also significantly expands upon its use. Ezjak teaches the introduction of per sulfate ions and oxygen into the sample, followed by irradiation of the sample with ultraviolet (UV) light. This causes the oxidation of organics in the sample and results in carbon dioxide production proportional to the organic content of the sample. The use of UV irradiation in this manner was disclosed by Regan in U.S. Pat. No. 3,958,941. The carbon dioxide gas is then transferred from the sample to a water stream as taught by Carlson.
Ezjak departs from the method of Carlson by then removing the gasses (carbon dioxide) from the water stream in a gas-liquid separator and transporting the gas to an infrared detector. The detector measures absorption of infrared light due to carbon dioxide and the concentration of carbon dioxide is then calculated from the amount of absorption.
Blades et al. discloses a method for measuring TOC which is intended to eliminate the calibration required of a system such as that described by Regan. Blades et al. utilizes a single chamber, transparent on a side, in which a sample is placed and its conductance measured. The sample is then irradiated by UV light while its conductance is continuously monitored. The TOC of the sample is determined from the change in conductance as the sample is irradiated.
Finally, Godec et al. discloses the most common current method for measuring TOC. In the method of Godec, a sample stream is first acidified to facilitate removal of inorganic carbon as carbon dioxide. This carbon dioxide can be transferred from the sample to a water stream and measured by conductive means as in Carlson. Oxidation catalysts are then added to the sample, which is then irradiated by UV light. The carbon dioxide produced by oxidation of the organic carbon is transferred to a water stream and measured conductively.
The most common method of measuring TOC uses elements of all these prior art patents. This method is used in most commercially available (UV)-persulfate analyzers and can be broken down into the following six steps:
1) Acid is injected to lower sample pH to below 4. This converts inorganic carbon from dissolved ionic forms to carbon dioxide gas. Inorganic carbon removal prevents interference with organic carbon detection. PA1 2) This carbon dioxide gas is picked up in a carrier gas stream and separated from the liquid sample in a gravity-dependent Gas-Liquid Separator (GLS). PA1 3) Oxygen is injected to oxidize the carbon present (in the form of water borne organic pollutants) to carbon dioxide gas. The oxygen is in the form of sodium persulfate. PA1 4) Carbon oxidation is enhanced using UV radiation. PA1 5) Carbon dioxide produced by oxidation is picked up in a carrier gas stream and separated from the liquid in a second gravity dependent GLS. PA1 6) The carbon dioxide, via the carrier gas, flows to an infrared (IR) detector where the TOC concentration is determined. PA1 1) Sample pH is controlled using a solid-phase acidifier instead of liquid acid injection. The solid phase acidifier allows control of acid addition and uses only the amount of acid that is necessary. The acidifier is consumed with usage and is sized for replacement at regular resupply intervals. PA1 2) Total Organic Carbon (TIC) separation uses a hydrophobic membrane. Membrane separation eliminates dependence on gravity, as well as liquid sparging, thereby avoiding significant loss of volatile organics. Carbon dioxide crosses the membrane and is purged from the GLS with dry, carbon dioxide-free oxygen. Simultaneously, oxygen diffuses across the membrane and provides the sample with essential oxygen for oxidation. PA1 3) UV oxidation, promoted by reactions between dissolved oxygen in the sample and incident radiation, eliminates the complexities of handling and dispensing liquid consumables. PA1 4) Membrane separation of organic carbon dioxide directly into the static Infrared (IR) cell provides maximum sensitivity, eliminates the need for carrier gas, and operates independent of gravity.